1. Field of the Invention
The present invention relates to a structure of an exhaust gas throttle valve provided in the exhaust system of an engine, such as a throttle valve disposed on the downstream of a device for purifying the exhaust gases of a diesel engine or an exhaust brake valve for assisting the braking force during the deceleration operation of a vehicle.
2. Description of the Related Art
As an important way to protect the environment, regulations have now been legislated for reducing pollution components contained in the exhaust gases emitted from the engines for vehicles, and a variety of technologies have been vigorously developed to reduce pollution components. In particular, the regulations against the exhaust gases of diesel engines are becoming stringent in recent years, and it is expected that more strict regulations will be legislated in the future. Diesel engines generally feature a higher thermal efficiency than that of the gasoline engines and emit carbon dioxide (CO2) in smaller amounts correspondingly. However, it has still been strongly demanded to reduce the emission of particulate matter (PM) and nitrogen oxides (NOx).
In order to prevent the emission of the particulate matter, there has been proposed a device for purifying the exhaust gases by mounting a filter called diesel particulate filter (DPF) in the exhaust system of a diesel engine to trap the particulate matter. The DPF usually comprises a ceramic body such as porous cordierite in which a number of fine passages are formed in the axial direction, the inlets and outlets of the neighboring passages being alternately closed. The exhaust gases of the diesel engine flow toward the downstream passing through the porous ceramic walls between the neighboring passages whereby the particulate matter in the form of fine particles is trapped. As the device for purifying the exhaust gases, the so-called continuously regenerating DPF is drawing attention in recent years. In this device, an oxidizing catalyst is disposed on the upstream of the DPF, the unburned components in the exhaust gases are oxidized to elevate the temperature of the exhaust gases, thereby to continuously oxidize and remove the particulate matter trapped by the DPF for preventing the choke of the DPF and regenerating the DPF while the engine is in operation.
In order for the catalyst in the continuously regenerating DPF to be activated and favorably regenerated, the exhaust gases must have been heated to an activating temperature that makes the catalyst to activate, i.e., must have been heated to not lower than about 350° C. During the low-load operation of the diesel engine where the fuel is injected in small amounts, however, the temperature of the exhaust gases becomes considerably low. If this operating condition continues for extended periods of time, the temperature of the catalyst becomes lower than the activating temperature, and the particulate matter deposits on the DPF. Even in the continuously regenerating DPF, too, therefore, the particulate matter must often be removed by activating the catalyst by intentionally elevating the temperature of the exhaust gases. The above regeneration of the continuously regenerating DPF is hereinafter referred to as forced regeneration.
At the time of the forced regeneration, the fuel is fed to the exhaust system by such means as post-injection to inject the fuel in the exhaust stroke of the engine to elevate the temperature of the exhaust gases, and is oxidized and burned in the catalyst. There has further been proposed means for more elevating the temperature of the exhaust gases by providing an exhaust gas throttle valve on the downstream of the continuously regenerating DPF and squeezing the exhaust passage when the forced regeneration is to be executed, as disclosed in, for example, JP-A-2003-343287. Here, a diesel engine having an exhaust gas throttle valve disposed on the downstream of the continuously regenerating DPF will be described with reference to a schematic diagram of FIG. 8, and a concrete constitution of the exhaust gas throttle valve will be described with reference to FIG. 7.
Referring to FIG. 8, the air is fed into the cylinders of a diesel engine body through an air cleaner 22 and an intake pipe 23. The fuel is injected into the cylinders from the fuel injection nozzles 24 at the end of the compression stroke, mixed with the compressed air, and burns in the cylinders to produce the power. The exhaust gases after burned are discharged into an exhaust pipe 25 and are partly recirculated into the intake pipe 23 through an EGR passage 26. The recirculation purposes chiefly for preventing the generation of NOx, and the amount of the exhaust gases recirculated is controlled by an EGR valve 27. In the exhaust pipe 25, for trapping the particulate matter in the exhaust gases, there is arranged a continuously regenerating DPF 28 equipped with a ceramic DPF 281 and an oxidizing catalyst 282 disposed on the upstream side. On the downstream of the continuously regenerating DPF 28, there is disposed an exhaust gas throttle valve 1 operated by a fluid pressure actuator 18. The exhaust gas throttle valve 1, by controlling the operation fluid introduced into the fluid pressure actuator 18 by using an electromagnetic valve 19, is maintained fully opened while the diesel engine is in operation.
In the continuously regenerating DPF 28, the oxidizing catalyst 282 is activated when the temperature of the exhaust gases from the diesel engine is not lower than a predetermined value, and the particulate matter trapped and deposited on the DPF 281 is oxidized with oxygen in the exhaust gases and is removed due to the exhaust gases elevated at a high temperature by the oxidizing action. When the diesel engine is operated with a low load for extended periods of time, however, the temperature of the exhaust gases drops, the activity of the oxidizing catalyst 282 drops, and the particulate matter is trapped and deposits in increased amounts. When the amount of deposition exceeds a predetermined amount, the temperature of the exhaust gases is elevated by such means as feeding the additional fuel by post-injection from the fuel injection nozzle 24 to perform forced regeneration of the DPF. The above control operation is executed by an instruction from an ECU 210 depending upon a pressure differential before and after the DPF 281, a temperature thereof, an engine rotational speed, etc.
In a low-load region of the engine where it is difficult to activate the catalyst even performing the post-injection, a operation is conducted to strongly squeeze the flow of the exhaust gases by decreasing the opening degree of the exhaust gas throttle valve 1 disposed on the downstream of the continuously regenerating DPF 28. This prevents the radiation of heat from the continuously regenerating DPF 28, the heat is retained therein, an increased load torque is exerted on the engine due to an increase in the engine exhaust gas pressure, the temperature of the exhaust gases is further elevated, and the oxidizing catalyst 282 is sufficiently activated to promote the regeneration of the DPF 281. Here, however, if the exhaust gas throttle valve 1 is squeezed, the operation of the diesel engine is seriously affected. Therefore, the forced regeneration which uses the exhaust gas throttle valve 1 in combination is so set as to be executed only when the vehicle is brought into a halt and the engine is in an idling state. In the foregoing was described the device for purifying the exhaust gases with the exhaust gas throttle valve 1 disposed on the downstream of the continuously regenerating DPF 28. However, the exhaust gas throttle valve 1 may be disposed on the upstream side as disclosed in JP-A-2003-343287 mentioned above.
Referring to FIG. 7, the exhaust gas throttle valve 1 is constituted as a butterfly valve arranged in an exhaust gas passage 2 of a valve casing 3. Insertion holes 8 for a valve shaft 5 are perforated in the walls of the exhaust gas passage 2, and both ends of the valve shaft 5 are fitted and supported therein via bushes. An intermediate portion of the valve shaft 5 penetrating through the exhaust gas passage 2 has a diameter greater than that at both ends thereof, and its upper portion is cut to form a flat surface over the whole width of the exhaust gas passage 2. Therefore, flanges 51 are formed among the intermediate portion of the valve shaft 5 and both ends thereof. Further, edge portions 9 are formed on the wall of the passage at positions where the insertion holes 8 for the valve shaft 5 are opened in the exhaust gas passage 2.
A plate-like valve body 4 of the butterfly valve is secured by bolts 6 to the flat surface of the intermediate portion of the valve shaft 5. The valve body 4 is of an elliptic shape close to a circular shape on a plan view, and the circumferential portion thereof comes nearly in agreement with the tubular wall of the exhaust gas passage 2 when the valve is closed. This figure illustrates fully opened state of the valve. To close the valve, the valve shaft 5 is turned by, for example, about 75 degrees by a link 7 coupled to the hydraulic pressure actuator to bring the circumferential portion of the valve body 4 close to the tubular wall of the exhaust gas passage 2. Here, however, the exhaust gas passage 2 is not completely closed even when the valve is at the fully closed position; i.e., a small gap is maintained between the circumferential portion of the valve body 4 and the tubular wall of the exhaust gas passage 2 enabling the diesel engine to be operated at the time when the DPF is to be forcibly regenerated.
Such exhaust gas throttle valve is used not only in combination with the continuously regenerating DPF but also as an exhaust brake valve as has been traditionally used, as disclosed in JP-A-11-257104. That is, many heavy duty trucks mounting diesel engines and having large gross vehicle weights are equipped with an exhaust brake valve of a similar constitution which works to greatly squeeze the exhaust gas during the deceleration, so that the load torque that acts on the diesel engines increases to assist the braking force during the deceleration of the vehicles for safety.